Estrogen and the estrogen-receptor antagonist tamoxifen therapy is associated with reduced risk of coronary artery disease events in postmenopausal women. Because of the contribution of inflammation to atherosclerosis, atheroprotective effects of estrogen and tamoxifen may in part be due to favorable changes in vascular responses to inflammatory mediators, as both estrogen and tamoxifen have been shown to have antioxidant properties in lipoprotein oxidation assays. In human coronary smooth muscle cells, tumor necrosis factor (TNF) alpha causes rapid increases in intracellular oxygen species, which are essential for transcription factor (NFkappaB) activation. NFkappaB in turn activates many genes leading to synthesis of proinflammatory proteins. We examined the effect of estrogen or tamoxifen on generation of reactive oxygen species using an intracellular marker (dichloroflourescein) and confocal laser microscopy for detection. One hour of pretreatment with 17beta-estradiol at physiological concentrations (0.1-10 nM) or tamoxifen at therapeutic concentrations (5-500 nM) dose-dependently inhibited TNF alpha-generated reactive oxygen species and inhibited binding of NFkappaB to DNA (by gel shift assay). Inhibitory effects on reactive oxygen species and NFkappaB binding to DNA were also seen with 17 beta-estradiol, a stereoisomer of estradiol that does not bind to the estrogen receptor. Hydrogen peroxide-generated reactive oxygen species were also inhibited by estrogen and tamoxifen. These early effects of estrogen and tamoxifen appear to require a hydroxyl group in position 3 of the A-ring, since 3-methoxy estrone that lacks this OH-group had no inhibitory effects on reactive oxygen species or on NFkappaB binding to DNA. Thus estrogen and tamoxifen may reduce coronary artery disease risk in part due to anti-inflammatory effects via reactive oxygen species scavenging, resulting in inhibition of proinflammatory gene activation.